Nezumi was raised from childhood by the yakuza to be an emotionless killer. Having never known love or a normal life, she exists as a tool for violence within the criminal underworld. Her life takes an unexpected turn when she meets Ao, an ordinary and somewhat hapless young man, during a hit at an arcade. Despite the blood on her hands, Ao pursues her earnestly, and the two begin a relationship that exposes Nezumi to the simple, tender moments of shared meals and intimacy she has never experienced.

The central conflict arises when this newfound happiness clashes with her brutal reality. Nezumi’s yakuza masters, who view her as their property, refuse to let her go. To protect Ao from being killed and to assert her own humanity, Nezumi is forced to make terrible choices, ultimately risking her own life to save him. The situation escalates when Ao, in an attempt to stay by her side, undergoes training and becomes a fellow assassin for the same organization. This leads to him being captured by a rival gang, forcing Nezumi to fight alone into enemy territory to rescue him.

The story is set in a realistic portrayal of Japan's criminal underbelly, contrasting violent locations with mundane settings like arcades and parks. The protagonist, Nezumi, is a young woman whose entire identity has been forged by abuse and murder, yet she begins to awaken to the possibility of a different life. Ao is the catalyst for her change, a regular boy whose sincere love offers Nezumi her first glimpse of normalcy and redemption.

The narrative is structured in notable arcs that began with their chance meeting and initial romance, followed by a section focused on the rising threat from the yakuza who attempt to reclaim her. As the story progresses, the couple attempts to integrate further into each other's dangerous worlds, culminating in a final arc where Nezumi must definitively choose between her past as a killer and her future with Ao, as the series enters its conclusion.